Method of manufacturing a flat steel product having an oxidation-resistant coating

ABSTRACT

A coating of oxidation-resistant material, having a given oxygen content, is deposited on mild steel sheet. The coated sheet is heat treated under a vacuum. Carbon already present in the coating and carbon diffusing into the coating combines with the oxygen in the coating, the products of this combination escaping because of the vacuum, so that the coating is de-oxidized and decarburized. To prevent chromium subliming because of the high temperature and low pressure, the sheet is heat treated as a compact stack or coil.

United States Patent [191 Liesenborghs et a1.

[ Dec. 16, 1975 METHOD OF MANUFACTURING A FLAT STEEL PRODUCT HAVING ANOXIDATION-RESISTANT COATING Inventors: Roland Cesar Amand Liesenborghs,

Sc1essin; Vincent Marguerite Leroy, Liege, both of Belgium Assignee:Societe Anonyme Cockerill-Ougree-Providence et Esperance-Longdoz,Seraing, Belgium Filed: Jan. 30, 1974 Appl. No.: 437,917

Foreign Application Priority Data Jan. 30, 1973 Belgium 794758 US. Cl.l48/l2.l; 148/14; 148/3l.5 Int. Cl. C21D 7/14 Field of Search 148/14,12.1, 127, 31.5

[56] References Cited UNlTED STATES PATENTS 2,470,988 5/1949 Kanter148/14 3,116,179 12/1963 Carpenter et a1.... 148/12.1 3,356,528 12/1967Gibson et a1 148/14 3,583,887 6/1971 Steger et a1 148/1 2.1

Primary ExaminerR. Dean Attorney, Agent, or FirmHo1man & Stern [57]ABSTRACT A coating of oxidation-resistant material, having a givenoxygen content, is deposited on mild steel sheet. The coated sheet isheat treated under a vacuum. C arbon already present in the coating andcarbon diffusing into the coating combines with the oxygen in thecoating, the products of this combination escaping because of thevacuum, so that the coating is de-oxidized and decarburized. To preventchromium subliming because of the high temperature and low pressure, thesheet is heat treated as a compact stack or coil.

" 4 Claims, No Drawings PRODUCT HAVING AN OXIDATION-RESISTANT COATINGThe present invention relates to a method of manu- 5 facturing a flatsteel product having an oxidation-resistant coating, the product being,for example, ordinary mild steel sheet. The coating comprises anoxidationresistant material which may consist of one or more of themetals Cr, Ni Co and M or of an alloy of two or more of these metalswith one another, or an alloy of one or more of these metals with ironor with other elements.

The most important use of oxidationresistant coatings concerns theprotection of ferrous products from corrosion and oxidation, and it hasalready been-recommended to use for this purpose powders comprising oneor more of the metals Cr, Ni, Co, and M0 or their alloys, whoseprotective properties are well-known.

Certain present-day processes consist in carefully cleaning thesubstrate, then depositing an alloy on the substrate by spraymetallization or by spreading and compacting the powders or bymetallizing through evaporation under vacuum, and finally subjecting thecoated substrate to a heat treatment at high temperatures in a dryhydrogen atmosphere in order to cause diffusion of the alloy.

After this treatment, the coated product may be rolled, andrecrystallization annealed, depending on the mechanical and structuralproperties required of the finished product.

Tests made on such products have shown that the surface finish of thecoatings as well as the quality of the corrosion resistance may beimpaired, owing to the two following phenomena.

In coatings provided on mild steel, for example by depositing an alloysuch as ferro-chromium, contamination by oxidation of the depositedalloy has been ob served. The diffusion heat treatment carried out indry hydrogen often proved unsuitable for reducing all the chromiumoxides present in the plating, particularly when the coated product wasin the form of a compact coil. The result is a deterioration in thesurface finish of the product thus treated.

With mild steels having a carbon content of approximately 0.050%, duringthe diffusion heat treatment in dry hydrogen there is also observed onthe one hand a migration into the substrate of the characteristicconstituents of the coating and on the other a migration of the carbonof the substrate into the coating, in which it can form chromiumcarbides. These chromium carbides present in the coating reduce thecorrosion resistance of the finished product. In order to avoid thismigration of the carbon of the substrate into the coating it has beenproposed to use steels which are stabilized with titanium. Theseadditions of titanium to the base steel result in the formation oftitanium carbides in the base steel, these carbides being stable at thetemperature of the diffusion treatment. It is also possible to avoidthis migration of the carbon by using decarburized steels.

The object of the present invention is to provide a method which allowsthese disadvantages to be removed without using titanium.

The complete manufacturing process, will normally comprise the variousstandard phases of cleaning the steel, depositing the selected metal oralloy, heat treating the deposit, and then, if required, rolling and re-2 crystallization annealing. For sheet steel of a given carbon contentand thickness the processinc'ludes:

1. making. a deposit of known oxygen content, to a thickness so that thetotal amount of oxygen is in i stoichiometric proportion to the amountof carbon available in the substrate and in the deposit, with a view toobtaining by a reaction between these .two elements during the diffusionheat treatment, substantially complete de-oxidation of the coating andde-carburization of the substrate and coating so that the formation of,for example, chromium carbide in the coating is negligible;

2. selecting a temperature and time for the diffusion heat treatmentwhich are capable of ensuring not only the above physico-chemicalreactions, but

also the desired diffusion of constituents of the deposit into the steelsubstrate;

3. carrying out this heat treatment under a sufficient vacuum in orderto ensure: the elimination from the coating of the products of thereaction of carbon with oxygen;

4. optionally, carrying out the heat treatment under vacuum on a compactcoil or stack of the coated sheet with a view to avoiding for examplethe removal of chromium from. the plating by sublimation at hightemperature, a phenomenon which has sometimes been observed as verysignificant.

It is preferable for the diffusion heat treatment to be carried out at atemperature of 950C to 1,300C for a time of 48 hours to 1/2 hour under aresidual pressure of 10 to 10 Torr.

EXAMPLE As a comparative example of a known technique, a

deposit of a Fe-72Cr alloy containing 0.020% C was formed on a steelsheet having a carbon content of 0.055% C and a thickness of 2 mm. Thethickness of the deposit was approximately l00 micrometers and itsoxygen content was 1.5%. According to the standard technique, adiffusion heat treatment of 16 hours at l,l50C in dry hydrogen of acompact stack of sheets resulted in the formation of a superficiallyoxidized coating having a carbon content of 0.0160% and a chromiumcontent of around 20%.

On the other hand, in accordance with the present invention, a depositof an Fe-72Cr alloy containing 0.020% carbon, having a thickness ofapproximately 100 micrometers and an oxygen content of 1.5% was formedon a steel sheet containing 0.075% C and having a thickness of 2 mm.After heat treatment at l,l50C for 12 hours under vacuum of 10 mm Hg ofa compact stack of sheets, a bright or brilliant coating was formedwhich had a carbon content of between 0.030 and 0.040% and a chromiumcontent of around tion-resistant material selected from the groupconsisting essentially of the metals Cr, Ni, Co, and Mo, alloys of atleast two of the metals with each other, and alloys of at least one ofthe metals with iron, the coating having an oxgen content of 1.5%;tightly winding the coated sheet to form a compact coil or stack of thecoated sheet; and heat treating the compact coil or stack of the coatedsheet at a temperature of 950-1300C for /2 hour to 48 hours under avacuum so 4 prising rolling the coated sheet.

4. The method as claimed in claim 3, further comprisingrecrystallization annealing the rolled coated sheet.

1. A METHOD OF MANUFACTURING A STEEL PRODUCT HAVING ANOXIDATION-RESISTANT COATING, COMPRISING THE STEPS OF: DEPOSITING ON AMILD STEEL SHEET A COATING OF AN OXIDATION-RESISTANT MATERIAL SELECTEDFROM THE GROUP CONSISTING ESSENTIALLY OF THE METALS CR, NI, CO, AND MO,ALLOYS OF AT LEAST TWO OF THE METALS WITH EACH OTHER, AND ALLOYS OF ATLEAST ONE OF THE METLS WITH IRON, THE COATING HAVING AN OXYGEN, CONTENTOF 1.5%; TIGHTLY WINDING THE COATED SHEET TO FORM A COMPACT COIL ORSTACK OF THE COATED SHEET; AND HEAT TREATING THE COMPACT COIL OR STACKOF THE COATED SHEET AT A TEMPERATURE OF 950-1300*C FOR 1/2 HOUR TO 48HOURS UNDER A VACUUM BEING SUFFICIENT SO THAT THE COATING IS COATING,THE VACUUM BEING SUFFICIENT SO THAT THE COATING IS DE-OXIDIZED ANDDECARBURIZED.
 2. The method as claimed in claim 1, in which the vacuumhas a residual pressure of 10 2 to 10 6 Torr.
 3. THE METHOD AS CLAIMEDIN CLAIM 1, FURTHER COMPRISING ROLLING THE COATED SHEET.
 4. The methodas claimed in claim 3, further comprising recrystallization annealingthe rolled coated sheet.